The presence of nickel sulphide is adventitious in most glass. It has been estimated that in normal float glass production NiS inclusions occur at a frequency of about one per 8 tonnes of raw glass It has been estimated that 1 gram of nickel sulphide can produce approximately 1000 inclusions of 0.15 mm diameter. Consequently, it does not require too much contamination to produce a serious problem.
NiS induced fracture in tempered glass has generally been found to be restricted to a batch of glass rather than all tempered glass. Numerous large high rise glass facades around the world have experienced NiS induced fracture. Yet in comparison there are literally thousands of high rise glass
facades that have not reported any fracture of tempered glass. The failure pattern is unique to each building. Some batches of NiS contaminated tempered glass experience glass breakage from the installation stage of the facade while others have an incubation period before failure commences.
Furthermore, failures have been known to occur up to 20 years after manufacture.
Nickel Sulphide inclusions often contain other metals such as iron, copper and cobalt. These impurities also generally influence the fracture behavior of the NiS inclusion.
The glass that is to be installed inside a framed glazing, point fixed infill panels, canopies, balustrades, external facades, should be tempered as well as heat soaked because these are the concern areas due to NiS induced glass failure which can cause:
• Personal injury due to falling glass:- An occupant or passer-by may be hit by falling fragments. When a pane of toughened glass falls spontaneously it produces a loud noise which may be heard from a significant distance away. The reaction of many people when hearing a noise above them is to look up, and this may result in fragments of glass striking a person in the face, or getting into the eyes.
• Result in unusable area: - An area of the building may be rendered unsafe or unusable until the glass can be replaced.
• Disruption in the office operation due to opening arising due to tempered glass failure which will leave a large hole on the facade of the building and may render the air conditioning system useless and office operation disturbed.
Heat Soak Production Process.
The heat soak process system consists of:
• The oven
• Glass support
• Separation system
The oven
As the standard is based upon extensive research combined with years of experience it was decided to limit the standard to ovens in which a batch of glass is heated up by
convection. Other methods of bringing the glass up to the required temperature e.g. micro-waves, immersion, etc... will be subject to further development and will require evidence before it can be accepted as a reliable alternative for the actual HST.
Moreover the oven shall be designed and constructed in such a way that it shall allow an
unhindered air circulation around each glass pane. Even in the event of glass breakage the airflow shall not be hindered. This will require a careful design of the openings for the air ingress/egress so that fragments of broken glass will not cause blockages.
Glass support
It is very important to lay down exactly the way the glass is put up inside the HST oven. The glass stack shall be aligned with the heating airflow direction.
Separation system
The distance between the single glass panes in the stack combined with the alignment of the glass stack are the most important parameters decisively determining the quality of the test. The separators shall assure optimized airflow between the glass panes without hindering the airflow themselves. A minimum separation of 20mm is recommended. An example of a vertical glass support is shown. It goes without saying that glass-to-glass contact is not allowed!
Distances between the glass panes of less than 20mm will not only badly influence the heat exchange but also increase the risk of breakage. The latter will certainly be the case when the oven is fully loaded as huge temperature differences can occur between 2 points in the same glass pane.
Specifications
The following test parameters have to be recorded:
• Time for the first thermocouple on a glass to reach a temperature of 280°C and the temperature of the control element at that time
• Maximum temperature of the control element during the heating phase and the time at which this temperature occurs
• Time for the last thermocouple on a glass to reach a temperature of 280°C
• Time of the control element and the glass surface temperatures
• Glass separation distance
• Separator position, material and shape
• Stillage(s) configuration
These records will allow to check if the heat soak process system meets the time/temperature requirements and gives the necessary information how to operate the HS oven during manufacture in accordance with the DIN 18516 (1).
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